Commonly owned U.S. Pat. Nos. 4,102,280 and 4,104,977 disclose and claim devices of this type designed to hold a workpiece, such as a stack of fabric layers to be stitched together along a linear seam, under a certain tension during transportation of that workpiece under a pressure foot of a sewing machine. The device comprises a trailing clamp reciprocable between a loading position and an unloading position, the clamp being entrained toward the unloading position by the engaged workpiece and being returned to its loading position by a counteracting force after releasing that workpiece.
According to U.S. Pat. No. 4,104,977, the counteracting force is provided by a weight; U.S. Pat. No. 4,102,280 discloses means for varying the counteracting force between a low value during forward motion and a high value during the return stroke, e.g. with the aid of a turbine driven by an air stream.
In systems of this type, wherein the workpiece is gripped by a moving clamp which is fluidically operated, the activating fluid must be supplied to the clamp-supporting carriage through a flexible conduit capable of following the movement of that carriage. Rubber hoses or the like used for this purpose have only a limited service life as they tend to fatigue upon repeated flexing and unflexing. Their elasticity varies with temperature and gives rise to unpredictable forces acting upon the clamp carriage, particularly when hoses are bent with a small radius of curvature. Finally, the loop formed by such a hose in certain carriage positions may interfere with the mobility of the operator and could also lead to entanglement with extraneous objects, particularly in a large plant comprising a multiplicity of closely juxtaposed sewing stations or the like.
Similar problems arise with fluid-actuated mobile clamps which are not entrained by the engaged workpiece but, on the contrary, are used to displace that workpiece over a predetermined path.